Fw: ATM Secondaries by Replication?

["Jim Miller" <jim@jtmiller.com>]

forgot to copy atm

----- Original Message -----
From: "Jim Miller" <jim@jtmiller.com>
To: "Bob May" <bobmay@nethere.com>
Sent: Sunday, August 26, 2001 11:36 AM
Subject: Re: ATM Secondaries by Replication?


hi bob

thanks for responding.  i'm not suggesting that the secondary have no
tolerance but that the tolerance could be relaxed since an angular
deflection over a shorter path results in a smaller displacement.  i'm just
trying to get a handle on objective rather than subjective measures to see
if a new design point might be possible.  i've no doubt that similar relaxed
figures on a primary would produce substandard images.

thoughts?

jtm

----- Original Message -----
From: "Bob May" <bobmay@nethere.com>
To: <atm@shore.net>
Sent: Sunday, August 26, 2001 10:10 AM
Subject: Re: ATM Secondaries by Replication?



The surface of the secondary does need to be fairly accurate as most of it's
surface is used to reflect the starlight to the focus point.  The light has
to be reflected all in the same direction so the surface does need to be
accurate.  Generally, if the surface is a sphere or cylinder of some long
radius, you get astigmitism effects at the EP to the amount of error you are
getting from that non-flat surface.  Random errors of flatness will make for
a softer focus of the image also.
I have a replicated mirror (6"F4) and it's barely a 1/4 wave surface and
that really isn't good enough as far as I am concerned for a flat and I
might note that there are what look like dust on the mold problems in the
surface also.  That would really affect the quality of the image.
I would suspect that for a $200 scope, replicated surfaces would be fine if
the scope was at least a 6" size but for quality work, I'd rather not use a
replicated surface.  OTOH, if you can do a 1/10th wave surface with no dust
or other problems, go for it and you'll sell a bunch.
Bob May
http://nav.to/bobmay
bobmay@nethere.com